A process known in the field as the Ryan-Holmes process employs the use of bottoms additives, such as a C4+ stream into the upper portion of a distillation column or the reflux condenser of the column, to enhance distillation separation and to save energy. The Ryan-Holmes process and modifications are described in part in U.S. Pat. Nos. 4,293,322, issued Oct. 6, 1981; 4,318,723, issued Mar. 9, 1982; 4,350,511, issued Sep. 21, 1982; 4,428,759, issued Jan. 31, 1984, now Reissue Pat. No. 32,600, reissued Feb. 16, 1988; 4,451,274, issued May 29, 1984; 4,462,814, issued Jul. 31, 1984; and 5,345,772, issued Sep. 13, 1994.
The principles of continuous distillation are described in Perry""s Chemical Engineer""s Handbook, Seventh Edition, McGraw-Hill, Section 13. FIG. 13-1 shows a schematic diagram for a simple distillation column with one feed, a rectifying section above the feed containing multiple stages of vapor/liquid equilibrium, an overhead condenser at the uppermost stage where heat is removed, a stripping section below the feed also containing multiple stages of vapor/liquid equilibrium, and a reboiler at the lowermost stage where heat is added to the system. FIG. 13-3 illustrates a complex distillation process where heat is removed from each stage of the rectifying section and heat is added to each stage of the stripping section.
This process of removing heat from one or more stages of the rectifying section, in addition to the overhead condenser, is known dephlegmation. A dephlegmator is thus a device that enables more than one stage of distillative rectification with the simultaneous removal of heat from each of those stages, without the withdrawal of liquid or vapor streams from the column. It may be used over the whole length of the rectification zone in a distillation column or on a selected zone.
Dephlegmators, which operate as rectifying and heat transfer devices in the gas processing field are well-known, and for example, are employed to separate helium, nitrogen, or helium and nitrogen mixtures from a natural gas stream. Some examples of dephlegmators-heat exchangers used for such separation processes include: U.S. Pat. Nos. 5,011,521, issued Apr. 30, 1991; 5,017,204, issued May 21, 1991; and 5,802,871, issued Sep. 8, 1998.
The invention relates to a separation and rectification process employing a dephlegmator-heat exchanger device and introducing a liquid additive to improve the rectification process.
The invention comprises a dephlegmator-heat exchanger process for the separation of a light gas component from heavy components in a feed gas stream, which process comprises rectifying the feed gas stream in a dephlegmator-heat exchanger to provide a lean component overhead gas stream and a rich liquid stream. The invention comprises introducing, for example, injecting, during rectifying, a selected amount of a liquid hydrocarbon additive stream, for example, into the top or an upper portion of the dephlegmator-heat exchanger; withdrawing the rich liquid stream with the additive from a lower portion of the dephlegmator-heat exchanger; and withdrawing the lean overhead gas stream from an upper portion of the dephlegmator-heat exchanger. Optionally, the liquid hydrocarbon additive recycled may be recovered from the liquid stream.
It has been discovered that the use of a liquid hydrocarbon additive in a dephlegmator-type heat exchanger in place of a conventional reflux condenser, has two primary benefits: the quantity of liquid hydrocarbon injected is considerably reduced; and for the same liquid hydrocarbon injection flow rate, the condenser temperature is increased. These effects can also be combined at intermediate values of flow and temperature.
The introduction of a liquid additive stream, typically a hydrocarbon stream, such as a C4+ stream, can increase the top or upper operating temperature of the dephlegmator-heat exchanger by at least 10xc2x0 F., for example, increasing the top temperature to about xe2x88x9230 to xe2x88x9240xc2x0 F. or more, rather than the usual temperature operating range of about xe2x88x9250 to xe2x88x92150xc2x0 F., to achieve a given separation of a hydrocarbon feed stream.
In the process, the dephlegmator-heat exchanger device employed may be represented by a heat exchanger whose construction and design, for example, cross-sectional area, permits the device to act as a rectifying distillation column and heat transfer device, wherein vapor flows upwardly, while condensed liquid flows downwardly. The vapor and liquid are in equilibrium in the device, so that several stages of rectification are developed, while each step has heat removed, and in effect, nonadiabatic distillation occurs.
Unlike a process using a condenser where heat is removed from the uppermost rectifying stage only, in the process of this application the dephlegmator is removing heat from more than the uppermost stage of a rectifying zone. The dephlegmator may remove heat from anywhere between two to every stage in the rectifying zone.
The process of the invention may be usefully employed in a variety of processes in the separation of gas feed streams, such as, but not limited to: the separation of acid gases, like carbon dioxide and hydrogen sulfide from methane; the recovery of ethane (C2H6) and propane (C3H8) from natural gas streams; the recovery of ethylene (C2H4) and propylene (C3H6) from refinery offgas streams; the recovery of ethylene or propylene in ethylene or propylene production plants; and the separation of hydrogen and carbon monoxide by liquid methane.
In many of the processes, the product to be recovered may be from either the overhead (lean) gas stream or the bottoms (rich) liquid stream from the dephlegmator-heat exchanger. The liquid additive may be removed from the bottoms liquid stream and recycled for use in the dephlegmator-heat exchanger, or alternatively, the bottoms liquid stream with the liquid additive may be directed for further processing or use. The illustrative process, as described, employs a single dephlegmator-heat exchanger; however, one or more dephlegmators-heat exchangers of the same or different design may be employed in series or parallel in any process, provided at least one of the dephlegmators-heat exchangers employs a liquid hydrocarbon additive stream. For example, with two dephlegmators-heat exchangers in series, the liquid additives of the same or different hydrocarbon compositions may be injected into one or both dephlegmators-heat exchangers to increase the temperature in each device and to aid the rectification and separation in each dephlegmator-heat exchanger.
The liquid additive introduced into the dephlegmator-heat exchanger may vary in composition and concentration, as required, to increase the dephlegmator-heat exchanger temperature levels and separation efficiency, depending on the particular rectification process carried out. For example, where the process is a carbon monoxide-hydrogen separation, the additive may comprise methane, while with other C2-C3 hydrocarbon separations, the liquid additive may comprise liquid hydrocarbon, or particularly, bottoms recovery products, like C4+ hydrocarbons, i.e., C4-C8, with C4, as the primary component preferred.
Generally, the liquid additive comprises a higher molecular weight hydrocarbon additive stream, which is generated in the particular process, or a by-product of the process, or is separately supplied. Usually, the liquid additive is introduced at the top or directly into an upper section of the dephlegmator-heat exchanger and may be introduced as a separate stream or be sprayed in particulate form into the rising vapor and falling liquid of the dephlegmator-heat exchanger. The amount of the liquid additive may range from about 200 mole percent of the feed gas, such as, from about 5 to 100 mole percent and at temperatures varying from up to 0xc2x0 F., e.g., xe2x88x92320 to xe2x88x9235xc2x0 F.
The process will be described for the purpose of illustration only in connection with certain illustrated embodiments; however, it is recognized that various changes, modifications, additions and improvements may be made by those persons skilled in the art of the invention, as described and disclosed, without departing from the spirit and scope of the invention.